Fibre optics are used in a variety of industries including aerospace, telecommunications, lasers and medical devices. A common problem involves switching lights on or off and switching light between paths or combinations of paths. A variety of solutions have been developed to fulfill these requirements including (i) beam splitters, (ii) shuttles, (iii) optical shutters and (iv) variation of the shutter concept, the twisted-nematic liquid crystal shutter.
A beam splitter in its most common form is a cube made from two triangular glass prisms, which are glued together at their base using Canada balsam. The thickness of the resin layer is adjusted such that for a certain wavelength half of the light incident through one “port” i.e. face of the cube is reflected and the other half is transmitted due to frustrated total internal reflection.
A shuttle will usually include an input pipe which moves to align with one of the outputs. A key feature of this switch is that all outputs are exclusive, so it cannot select more than one at a time. Whilst it is possible to create an intermediate position for the shuttle such that it shines light into two outputs, considerable light will be lost at this junction owing to differences in the geometry of the input and outputs. As it stands, there is no “off” position. If one were needed separate from the light source itself, it would either have to separate, or include a dummy switch position.
Common shutter mechanisms include a blade, which may be introduced into a light path to block the transmission of light or rotated out of the light path to allow transmission. The shutter may be spring loaded and attached to a driver such as a rotary solenoid such that the blade moves to the energized position when it receives an operating voltage and returns to its resting position when the voltage is removed. Alternatively, manual operation of the shutter is possible.
The shutter mechanism relies entirely upon a simple mechanical beam blocking effect. It is inefficient as this light is “lost”. Furthermore, the “lost” light may be converted to heat, which is undesirable in some applications. Excessive local accumulation of heat can lead to burns in medical device applications where the device is in contact with the patient or user.
Liquid crystal displays provide for another type of shutter: the Twisted-Nematic Liquid Crystal Shutter.
The twisted nematic effect (TN-effect) is the breakthrough that made liquid crystal displays practical in portable devices and allowed them to replace technologies such as light emitting diodes and electroluminescence from most electronics.
TN-cells do not require a current to flow for operation and use low operating voltages suitable for use with batteries. The twisted nematic effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. This is achieved with little power consumption and at low operating voltages.
In one example, a TN-cell in the OFF state, i.e., when no electrical field is applied, a twisted configuration of nematic liquid crystal molecules is formed between two glass plates, which are usually separated by several spacers and coated with transparent electrodes.
The electrodes themselves are coated with alignment layers that precisely twist the liquid crystal by 90° when no external field is present. When light shines on the front of the LCD, light with the proper polarization will pass through the first polarizer and into the liquid crystal, where it is rotated by a helical structure. The light is then properly polarized to pass through the second polarizer set at 90° to the first. The light then passes through the back of the cell and the image appears transparent.
In the ON state, i.e., when a field is applied between the two electrodes, the crystal realigns itself with the external field. This “breaks” the careful twist in the crystal and fails to re-orient the polarized light passing through the crystal. In this case the light is blocked by the rear polarizer and the image appears opaque.
The degree of opacity can be controlled by varying the voltage; at voltages near the threshold only some of the crystals will re-align, and the display will be partially transparent, but as the voltage is increased more of the crystals will re-align until it becomes completely “switched”. A voltage of about 1 V is required to make the crystal align itself with the field, and no current passes through the crystal itself. Thus the electrical power required for that action is very low.
The obvious advantage such TN-cell shutters have is that they may be operated at very high switching speeds and with low operating voltage. For example switch speed of less than 0.3 milliseconds is typical at room temperature with an applied voltage of only 10V.
Furthermore, activation or switching speed can be enhanced via use of higher operating voltages.
However, the technology has several limitations. Notably, for unpolarised light with 500 nm wavelength (the approximate mid-point of the visible spectrum), transmission of light does not exceed 35% in the ON position, meaning that considerable light is lost. Furthermore, when the device is in the OFF position there is still some light transmitted. Even although the amount of light transmitted is typically less than 0.5% it is not completely blocked as with a purely mechanical shutter mechanism.
Moreover, a long term DC component in the voltage will stimulate impurity ion migration and eventual failure of the device. Therefore such devices have a finite useful lifetime.
Further, caution must be exercised in the handling and cleaning as it is easy to accidently damage the polariser surface or its components, by accidental scratching, use of inappropriate cleaning materials or even simple over-exposure to sunlight.